I see I need to enlighten some people about the gates, and what exactly they are, and what they do.
The eight gates are chakra "gates" in the chakra circulatory system that restrict the overal flow of chakra in the body.
Ninjas normally use only 20% of their total chakra at an given time. This isn't to say that they can't run out of chakra, as long battles will eventually wear them down, but they cannot forcibly use all of their chakra at any one time without opening the gates.
There are 8 gates, each gate allowing access to 10% more of the user's total chakra potential. Upon opening the 8th gate the user has full access to all of their chakra, and that is why it named the gate of Death, because what happens when you use all of your chakra? You die.
The gates does not magically increase your strength or speed, what it does is allow you to augment your speed and strength with chakra. Every character does this, such as the raikages, Naruto, Sasuke, anyone who uses shunshin, everyone. Tsunade does it for strength, as does Sakura, and every new mode (Sage Mode, KM, BM, raiton armor) is basically doing the same thing.
The gates are harmful because the users body is unused to the strain of having their muscles pushed that hard because of the full access of their chakra.
My overall point is that the 8 Gates are overestimated. It is limited by the users chakra levels, their chakra control (as better chakra control results in more strength or speed), and their speed and strength before opening the 8 Gates.
This is why Guy (and Lee) will never be as good as someone like the raikages, or Tsunade, or Jiraiya, or Orochimaru. He doesn't have the chakra levels for it, and so can't hope to match them.
I mean this is a retarded quote, "Upon opening the 8th gate the user temporarily gains powers greater than a kage." What does it even mean? Not all kages are taijutsu fighters, and not all kages are the same level. Does this mean that somehow Guy becomes temporarily greater than every kage to ever exist? Does he surpass Hashirama, Hiruzen, Tobirama, and Minato?
Instead of taking every single statement about power literally, we should instead look at how the abilities work, their limits, and strengths.